Process for wafer backside polymer removal with a ring of plasma under the wafer

ABSTRACT

A process is provided for removing polymer from a backside of a workpiece. The process includes supporting the workpiece on the backside in a vacuum chamber while leaving at least a peripheral annular portion of the backside exposed. The process further includes confining gas flow at the edge of the workpiece within a gap at the edge of the workpiece on the order of about 1% of the diameter of the chamber, the gap defining a boundary between an upper process zone containing the wafer front side and a lower process zone containing the wafer backside. The process also includes providing a polymer etch precursor gas underneath the backside edge of the workpiece and applying RF power to a region underlying the backside edge of the workpiece to generate a first plasma of polymer etch species concentrated in an annular ring concentric with and underneath the backside edge of the workpiece.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/898,645, filed Jan. 30, 2007.

BACKGROUND

Plasma processing of a workpiece or semiconductor wafer, particularlydielectric etch plasma processing, typically employs carbon-containingprocess gases (e.g., fluorocarbon or fluoro-hydrocarbon gases) thatenhance the etch selectivity of dielectric materials, such as silicondioxide, relative to other materials such as silicon. These processesare used to treat the front (top) side of the wafer on which themicroelectronic thin film structures are formed. The opposite (back)side of the wafer is typically unpatterned. One problem is that thecarbon-containing process gases tend to form polymer precursors in theplasma, which can leave a polymer residue on the front side of the waferand on the exposed portion of the backside of the wafer, and even somedistance under the unexposed portion of the wafer backside. Suchresidues should be removed to avoid contamination of later processingsteps. The polymer residues deposited on the wafer front side tend to beeasily removed with plasma ion bombardment using appropriate chemistry.However, the wafer edge is beveled, and the curved surface on thebackside of the wafer edge is also exposed and therefore susceptible topolymer deposition during plasma processing. The backside of the curvedsurface of the wafer edge is shadowed from ion bombardment during plasmaprocessing so is more difficult to remove, but can be removed in anoxygen plasma at high temperature (e.g., above 300 degrees C.). Suchdifficult-to-remove polymer films require a post-etch polymer removalstep using (for example) an oxygen-rich plasma for thorough polymerremoval.

In many applications, the plasma etch process is used to form openings(e.g., trenches or contact holes) through multiple thin films on thewafer front side. Such thin film structures can include (for example) aspecial carbon-containing dielectric film having an ultra-low dielectricconstant (ultra low-K film). The ultra low-K film is exposed incross-section at the side wall of each trench or contact opening formedby the etch process step. Attempting to remove the backside polymer filmby heating and exposing the wafer to an oxygen-rich plasma (during apost-etch polymer removal step) will damage the ultra low-K film byremoving carbon from it. In semiconductor structures having 60 nmfeatures sizes (or smaller), such damage to the ultra-low K film ispermitted only to a depth of about 3 nm beyond the exposed surface(e.g., 3 nm beyond the sidewall of the opening). In contrast, thepolymer film deposited on the wafer backside edge is about 700 nm thick.It is generally difficult if not impossible to avoid damaging the ultralow-K (ULK) film beyond the permissible 3 nm depth while exposing thewafer to an oxygen-rich plasma of a sufficient density and for asufficient time to remove 700 nm of polymer from the backside of thewafer edge or bevel. The required polymer-to-ULK film etch selectivity(over 200:1) for such a polymer removal process in general cannot bemaintained reliably in conventional processes.

In conventional plasma reactor chambers, the wafer support pedestalincludes an annular collar surrounding the edge of the wafer. Such acollar tends to shield the wafer edge, but cannot be sufficiently closeto the wafer edge to prevent polymer deposition on the backside of thewafer edge. This is because some finite gap between the wafer edge andthe collar is required to accommodate variations in the robot waferplacement and tolerance stackup. Moreover, the wafer edge-to-collar gaptends to increase as successive wafers are etched in the chamber, sincethe collar is (typically) formed of a process-compatible material (e.g.,quartz, silicon or silicon carbide) that is gradually etched away duringplasma etch processing of successive wafers. Therefore, it has seemedinevitable that unwanted polymer is deposited on the wafer, includingthe backside edge of the wafer.

The foregoing problems might be avoided by using a rich mixture ofoxygen in the plasma during the initial etch process. However, thisapproach is not practical if the thin film structure on the waferincludes an ultra-low K film that is exposed on a sidewall of an etchedopening. Such a rich oxygen mixture in the etch plasma would causeunacceptable damage to the ultra-low K film.

There is a need for a way of removing polymer from the backside of thewafer (i.e., the backside of the wafer edge) without harming or damagingany low-K film layers in thin film structure.

SUMMARY OF THE INVENTION

A process is provided for removing polymer from a backside of aworkpiece. The process includes supporting the workpiece on the backsidein a vacuum chamber while leaving at least a peripheral annular portionof the backside exposed. The process further includes confining gas flowat the edge of the workpiece within a gap at the edge of the workpieceon the order of about 1% of the diameter of the chamber, the gapdefining a boundary between an upper process zone containing the waferfront side and a lower process zone containing the wafer backside. Theprocess also includes providing a polymer etch precursor gas underneaththe backside edge of the workpiece and applying RF power to a regionunderlying the backside edge of the workpiece to generate a first plasmaof polymer etch species concentrated in an annular ring concentric withand underneath the backside edge of the workpiece.

The process can further include removing polymer etch species from theupper process zone. In one embodiment, removing polymer etch speciesfrom the upper process zone is carried out by pumping a purge gas into acenter portion of the upper process zone while evacuating the upperprocess zone through a slit opening at the periphery of the upperprocess zone. The purge gas may be either a non-reactive species or areactive scavenger species.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of theinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings. It is to be noted, however, that the appendeddrawings illustrate only typical embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

FIG. 1A depicts a backside polymer removal reactor chamber in whichpolymer etch species are furnished from a first external plasma sourcetoward the backside of the wafer.

FIGS. 1B and 1C are plan and elevational views, respectively, of animplementation of the workpiece support pedestal in the reactor of FIG.1A that can be used in each of the reactors described herein.

FIG. 2 depicts a modification of the backside polymer removal reactorchamber of FIG. 1A in which etchant scavenger species are supplied froma second external plasma source toward the front side of the wafer.

FIG. 3 depicts another backside polymer removal reactor chamber in whicha concentrated stream of hot radicals or ions are directed to the waferbackside edge from a separate plasma source that is near the wafer.

FIG. 4 is an enlarged view of a portion of the chamber of FIG. 3depicting the placement of special materials to contain the concentratedstream of hot radicals or ions.

FIG. 5 depicts a backside polymer removal process carried out with thereactor chamber of FIG. 1A.

FIG. 6 depicts a backside polymer removal process carried out with thereactor chamber of FIG. 2.

FIG. 7 depicts a group of additional steps for the process of FIG. 6 forremoving photoresist from the wafer front side.

FIG. 8 depicts an alternative group of additional steps for the processof FIG. 6 for removing photoresist from the wafer front side.

FIG. 9 depicts a backside polymer removal process carried out with thereactor of FIG. 3.

FIG. 10 depicts an alternative process carried out in the reactor ofFIG. 2 for simultaneously removing backside polymer and removingfrontside photoresist from the wafer.

FIG. 11 depicts a modification of the reactor of FIG. 3 in which theexternal plasma source of the plasma stream is replaced by an internalinductively coupled source.

FIG. 12 depicts a modification of the reactor of FIG. 11, in which theinternal inductively coupled source is replaced by an internalcapacitively coupled source.

FIG. 13 depicts an alternative approach in which a ring plasma isgenerated beneath the wafer backside edge by an inductively coupledsource.

FIG. 14 depicts a modification of the reactor of FIG. 13 in which theinductively coupled source is replaced by an internal capacitivelycoupled source electrode for generating the ring plasma.

FIG. 15 depicts a modification of the reactor of FIG. 14 in which theinternal capacitively coupled source electrode is replaced by anexternal capacitively coupled source electrode.

FIG. 16 depicts a feature of the ceiling for shielding the wafer frontside during backside polymer removal.

FIG. 17 is a block diagram of a process for backside polymer removal andfront side photoresist strip involving temperature switching.

FIG. 18 illustrates a first reactor adapted to perform the process ofFIG. 17.

FIG. 19 illustrates a second reactor adapted to perform the process ofFIG. 19.

FIGS. 20 and 21 depict a modification of the reactor of FIG. 2.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The drawings in the figures are all schematic and not toscale.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the invention pertain to removing polymer fromthe backside edge of a wafer without damaging critical films, such as anultra low-K dielectric film, by heating the wafer in a chamber whileexposing only the backside of the wafer to polymer etch radicals orplasma by-products, such as atomic or free oxygen, from an externalplasma source. The oxygen radicals may be provided by an external plasmasource which is supplied with an oxygen-containing gas or vapor, such asO2, H2O, N2O, CO2, or CO, for example. The oxygen-containing gas may becombined or diluted with other gases such as H2, N2 or Ar. Otherfluorine-containing gases (such as CF4 or NF3) may be added to allowremoval of polymer films that contain other materials (such as Si) andare not etched efficiently in O chemistry alone. The critical films inthe thin film structure on the wafer front side are protected fromdamage by the polymer etch species by pumping purge gases across thewafer front side. In addition, the wafer edge and the chamber side wallare separated by a very narrow gap to define a lower process zone withthe wafer backside and an upper process zone with the wafer front side.The narrow of the gap is configured to resist or minimize migration ofpolymer etch species from the lower to the upper process zone where itwould attack the ultra low-K film on the wafer front side. The externalplasma source is coupled to the lower process zone so that the polymeretch species are delivered to the wafer back side. Delivery rate andresidence time of polymer etch species are minimized in the upperprocess zone by restricting the height of the upper process region to avery narrow gap between the wafer and the chamber ceiling. The purgegases pumped across the wafer front side may be inert or non-reactive.Such purging reduces the etch rate of the front side critical filmsrelative to the backside polymer removal rate.

In one embodiment, to further reduce the etch rate of the critical filmsrelative to the backside polymer removal rate, the purge gases may besupplemented with or replaced by reactive scavenger gases thatchemically scavenge the backside polymer etch species in the upperprocess region.

In another embodiment, a further increase in the backside polymer etchrate is attained by employing a second (upper) external plasma sourcecoupled to the upper process zone. Gases that are precursors of speciesthat scavenge the polymer etchant are introduced into the upper externalplasma source, to produce scavenger radicals for the upper process zonethat reduce the amount (partial pressure) of polymer etchant species inthe upper process zone. In one embodiment, pressure is maintainedsufficiently low pressure in the upper process zone to enable the upperexternal plasma source to generate plasma efficiently, whilesimultaneously achieving a sufficiently high flow rate of scavengerspecies to protect the wafer front side thin films. In anotherembodiment, unused polymer etch species are removed from the lowerprocess zone before they migrate to the upper process zone. In oneembodiment, the upper and lower process zones are separately evacuatednear the wafer edge through separate pumping evacuation ports in thesidewall near the wafer edge. In addition, the scavenger species fromthe upper external plasma source may be heated before they enter theupper process zone.

The scavenger species furnished by the upper external plasma source thatremoves the polymer etch species from the upper process zone (e.g.,hydrogen) may also serve to remove photoresist from the wafer frontside. In such a case, photoresist removal may be performed in a separatestep in which no polymer etch species are introduced into the lowerprocess zone and the wafer-to-ceiling gap (height of the upper processzone) is increased. In addition, an agent gas (e.g., nitrogen) thatpromotes the etching of photoresist from the wafer frontside may befurnished in a small quantity to the upper external plasma source. In analternative mode, none of the front side thin film layers is susceptibleto damage by the polymer etch species, and the steps of backside polymeretch removal and front side photoresist removal are performedsimultaneously using the upper and lower external plasma sources. Inthis case, the height of the upper process zone is increased by loweringthe wafer support pedestal.

In one embodiment, an increase in polymer etch rate relative to etchrate of the critical (ultra low-K) film by the polymer etch species isachieved by locating the lower external plasma source very close to thewafer edge and directing a concentrated stream or jet of plasma productsfrom the lower external plasma source directly at the wafer backsideedge while rotating the wafer. By reducing the lower external plasmasource pressure, the concentrated stream may consist of polymer etchantions, radicals and neutrals, while at a higher pressure the streamconsists of etchant radicals and neutrals.

Referring now to FIG. 1A, a plasma reactor for removing polymer residuefrom the backside of a semiconductor wafer includes a reactor chamber100 having a side wall 102, a ceiling 104 that is a gas distributionplate and a floor 106. The ceiling or gas distribution plate 104 has aninterior gas manifold 108 and plural gas injection orifices 110 openingfrom the manifold 108 into the interior of the chamber 100. A wafersupport pedestal 112 in the form of a disk-shaped table has a diameterless than the diameter of a workpiece to be supported on the pedestal112, so as to expose the backside peripheral annulus of the workpiece.The pedestal 112 is supported on a lift member 114 elevated and loweredby a lift actuator 116. A workpiece such as semiconductor wafer 118 canbe supported with a center portion of its backside resting on thepedestal 112. The front side of the wafer 118 (the side on which themicroelectronic thin film structures are formed) faces the ceiling gasdistribution plate 104. The pedestal 112 is sufficiently small to leavean annular periphery of the wafer backside exposed, for backside polymerremoval. Often, the wafer 118 has a rounded or beveled edge as depictedin FIG. 1A. Such a beveled feature may make it difficult to avoidpolymer deposition on the wafer backside during plasma (e.g., etch)processing of thin films on the wafer front side. In one embodiment,radial arms 113 are provided to position the workpiece 118 on thepedestal 112. In one embodiment, three symmetrically spaced thin radialarms 113 shown in FIG. 1B are provided and extended outwardly from theperiphery of the pedestal 112. As shown in FIG. 1C, the radial arms 113are below the workpiece support surface of the pedestal 112 so as toleave the entire peripheral annulus of the workpiece backside exposedfor backside polymer removal. Each radial arm 113 supports a thin axialtab 113 a at its distal end, the tabs 113 a serving to locate theworkpiece 118 in coaxial alignment with the pedestal.

The chamber side wall 102 can include a removable liner or process kit120. Hereafter, the term sidewall 102 is used to include a liner 120 ifone is present. A gap 122 between the wafer edge 118 a and the side wall102 is very small, e.g., about 0.2-2 mm, so as to resist migration ofgases through the gap 122. The gap is configured to be sufficientlynarrow to present a gas flow resistance that is on the order of aboutone hundred times greater than gas flow resistance in other portions ofthe chamber. The gap 122 may be on the order of about 1% of the chamberdiameter. In this way, the wafer 118 divides the chamber 100 into anupper process zone 130 bounded in part by the front side or top surfaceof the wafer 118 and a lower process zone 132 bounded in part by thebackside or bottom surface of the wafer 118. A bottom external plasmasource 134 receives a polymer etch precursor gas from a gas supply 136and furnishes polymer etch radicals (e.g., oxygen radicals or atomicoxygen) into the lower process zone 132 through a port 138 in thechamber floor 106.

Some polymer etchant (e.g., oxygen) radicals can migrate from the lowerprocess zone 132 into the upper process zone 130 through the gap 122 andpose a risk of damage to critical layers on the wafer front side, suchan ultra low-K thin film. In order to prevent this, a non-reactive purgegas, namely a gas that does not react with the thin film materials onthe wafer front side, (e.g., nitrogen gas or argon gas) is supplied tothe ceiling gas distribution plate 104 from a gas supply 140 in order toflush out the upper process zone 130 and keep it free of etchantspecies. In order to facilitate a thorough and fast purge of the upperprocess zone 130, the upper process zone 130 is restricted to a verysmall height corresponding to a small wafer-to-ceiling gap 144, e.g.,about 0.2-2 mm. The gap 144 may be sufficiently small to confine thecross-section of the upper zone 130 to an aspect ratio greater than 100.The upper process zone height (gap 144) is sufficiently small so thatresidence time of gas in the upper process zone 130 is less than aboutone tenth to one hundredth of the gas residence time in the lowerprocess zone 132. Also, the upper process zone height (gap 144) issufficiently small so that gas flow resistance through the gap 144 is onthe order of 100 times the gas flow resistance through the lower processzone 132. Such confinement of the upper process zone height may beaccomplished by raising the wafer support pedestal 112 with the liftactuator 116.

In one embodiment, pressure in the chamber 100 is controlled by a vacuumpump 146 that maintains the lower process zone 132 at a sufficiently lowpressure to draw plasma by-products out of the lower external plasmasource 134 and maintain the external plasma source 134 at a sufficientlylow pressure to enable it to efficiently generate plasma. Alternatively,the upper and lower process zones 130, 132 may be evacuated separatelythrough separate slit openings near the wafer edge by separate pumps210, 216. In this case, the vacuum pump 146 may not be necessary.

In one embodiment, the polymer removal process is quickened by heatingthe wafer 118 to on the order of 300 degrees C., for example, either byan electrical heating element 150 inside the pedestal 112 or by radiantlamps (not shown). An electrical heater power supply 152 is coupled tothe heating element 150 through wires in the lift member 114. By raisingthe wafer temperature to about 300 degrees C., the backside polymerremoval rate is significantly increased.

In one embodiment, etching of films (e.g., an ultra low-K film) on thewafer front side is minimized or eliminated by maintaining a very highflow rate of a non-reactive purge gas (e.g., nitrogen or argon) throughthe ceiling gas distribution plate 104 into the upper process zone 130.This improves the polymer etch selectivity, i.e., the ratio of thepolymer etch rate to the ultra low-K film etch rate. The purge gas flowrate may be as high as necessary to achieve a desired etch selectivity,raising the pressure of the upper process zone 130 to a very high level.The pressure in the lower process zone 132 is maintained at asufficiently low level (e.g., a few Torr or less) to ensure efficientoperation of the external plasma source 134. In order for the externalplasma source 134 to generate a plasma, the chamber interior pressure ofthe external plasma source 134 typically should not exceed a few Torr,and because the external plasma source 134 is coupled directly to thelower process zone 132, the pressure in the lower process zone 132should be maintained at a correspondingly low level. This requirement ismet by the main chamber vacuum pump 146 (or by the vacuum pump 216)regardless of the high flow rate of purge gas into the upper processzone 130 through the gas distribution plate 104. This permits the purgegas flow rate and upper process zone pressure to be as great asnecessary to eliminate or minimize etching of any ultra low-K film onthe wafer front side.

In one embodiment, to further increase the rate of polymer removal fromthe wafer backside, a dissociation agent gas (e.g., nitrogen) thatpromotes dissociation of the polymer etch precursor gas (e.g., oxygen)may be supplied at a low flow rate to the external plasma source 134from a gas supply 156.

In one embodiment, to further reduce the amount of polymer etch species(e.g., oxygen) in the upper process zone 130, a gas supply 158 furnishesan etchant scavenger gas (e.g., hydrogen or carbon monoxide) to theceiling gas distribution plate 104. This may be instead of or inaddition to the non-reactive purge gas from the gas supply 140. Somepolymer etchant (e.g., oxygen) atoms or molecules that have migratedinto the upper process zone 130 are chemically consumed by combiningwith the scavenger gas (e.g., H2 or CO). Optionally, this scavengerreaction may be accelerated by heating the scavenger gases furnished tothe gas distribution plate 104 with an electrical heater 159. If thepolymer etchant precursor gas is oxygen, then the scavenger gas may becarbon monoxide or hydrogen. Carbon monoxide is less reactive with acarbon-containing ultra low-K film than the oxygen gas that it scavengesfrom the upper process zone 130. Hydrogen gas may be a good choice forthe scavenger gas because it may not deplete carbon from thecarbon-containing ultra low-K film, and therefore fulfills a requirementof being less reactive with the ULK film than the polymer etch species(oxygen) that it scavenges. The scavenger gas is selected so that theproduct of the chemical reaction between the scavenger and the polymeretchant (e.g., oxygen) does not react at a high rate with the ultralow-K film. In the case of a hydrogen scavenger and oxygen as thepolymer etchant, the product is water and in the case of a carbonmonoxide scavenger, the product is carbon dioxide, satisfying therequirement of a scavenging reaction product that is safe for the ultralow-K film.

In an optional mode, the reactor of FIG. 1A is used to removephotoresist from the wafer front side. In this mode, the wafer pedestal112 may be lowered to the dashed-line position of FIG. 1A, to enlargethe upper process zone 130 with a wafer-to-ceiling gap of about 0.5 to 5cm. If none of the thin film materials on the wafer front side include aULK film or other material susceptible to damage from oxygen, then theoxygen radicals from the lower process zone 132 are permitted to migrateinto the upper process zone 130 by halting the purge gas flow from thegas supply 140 and/or the scavenger gas flow from the gas supply 158.Also, such migration may be enhanced if the sidewall-wafer gap 122 isgreater at the lowered (dashed-line) wafer position. In this optionalmode, the backside polymer and the frontside photoresist are removedsimultaneously.

FIG. 2 depicts a modification of the reactor of FIG. 1A. The reactor ofFIG. 2 may be particularly advantageous in providing even greaterprotection for ultra low-K films on the wafer front side during backsidepolymer removal. In the reactor of FIG. 2, a second external plasmasource 200 is provided. The amount of polymer etchant species (e.g.,oxygen) in the upper process zone 130 is reduced more efficientlybecause the second (upper) external plasma source 200 provides plasmaby-products (e.g., radicals) of a scavenger species (e.g., hydrogen ornitrogen) to the ceiling gas distribution plate 104. The scavengerspecies radicals (hydrogen or nitrogen) chemically scavenge or combinewith some polymer etchant species (e.g., oxygen) that may migrate intothe upper process zone 130. Such radicals tend to have a faster rate ofreaction with the etchant species (than the molecular gas scavenger ofthe reactor of FIG. 1A) and therefore provide a higher rate of removalof etchant species (e.g., oxygen) from the upper process zone 130. Thisprovides superior protection from attack upon thin film structures(e.g., an ultra low-K film) on the wafer front side. A gas supply 202furnishes a molecular gas form of a scavenger precursor (e.g., hydrogenor nitrogen gas) to the chamber of the top external plasma source 200.By-products of the plasma generated by the source 200 (e.g., eitherhydrogen radicals or nitrogen radicals) are scavengers of the polymeretch species (e.g., oxygen), and are delivered to the gas distributionplate 104 to reduce or eliminate oxygen from the upper process zone 130and thereby protect the thin film structures on the wafer front side.

As depicted in FIG. 2, the top external plasma source 200 may consist ofa dielectric (e.g., quartz) tube chamber 204 encircled by an RF coilantenna 206 that is driven by an RF plasma source power generator 208through an impedance match element 209. The quartz material iscompatible with the hydrogen or nitrogen chemistry of the top externalplasma source 200. The top external plasma source 200 produces radicalsof the scavenger species that are fed to the ceiling gas distributionplate 104 through a center port 212. In order to provide uniformdistribution of the scavenger species across the gas distribution plate,a baffle 214 is provided in the center of the gas manifold 108 thatblocks direct gas flow from the center port 212 to gas injectionorifices 110 near the center of the gas distribution plate 104. Atoroidal plasma chamber may be used for either (or both) the top andbottom external plasma sources 200, 134. Such a toroidal chamberconsists of reentrant conduit of a conductive material. To accommodatethe hydrogen chemistry used in the top external plasma source 200, sucha toroidal plasma chamber may include an insulating liner protecting theconductive chamber or conduit.

The chamber pressure within the top external plasma source 200 should besufficiently low (e.g., not exceeding several Torr) to ensure efficientplasma generation within the external chamber 204. Since the topexternal plasma source 200 is coupled to the upper process zone 130(through the gas distribution plate 104), the upper process zonepressure cannot be too high without extinguishing plasma in the externalsource 200. Meeting this limitation may prevent a sufficiently high flowrate of scavenger species into the upper process zone 130 necessary toprotect the wafer front side. In one embodiment, an upper zone vacuumpump 210 is coupled directly to the upper process zone 130 through anupper zone vacuum slit passage 217 that is near (but slightly above, bya few mm or less) the wafer edge and passes through the side wall 102(and liner 120 if present). The upper zone vacuum pump 210 facilitatesor ensures a sufficient flow rate through the very thin wafer-sidewallgap 122. With this feature, the main vacuum pump 146 may be eliminatedin the reactor of FIG. 2, as will be discussed below. In one embodiment,the pumping rate of the upper zone pump 210 is maintained at asufficient level to keep the pressure in the upper process zone 130below a few Torr, for example. This permits a high flow rate of radicalsfrom the top external plasma source 200 and ensures quick removal ofscavenger-etchant reaction by-products from the upper process zone 130.The present embodiment provides a low chamber pressure inside the topexternal plasma source 200 to facilitate efficient plasma generationwithin the top external plasma source 200. The present embodiment alsoreduces the migration of scavenger or purge species into the lowerprocess zone 132 that would otherwise dilute the polymer etch species(e.g., oxygen) at the wafer backside.

In one embodiment, to reduce migration of polymer etchant species (e.g.,oxygen) from the lower process zone 132 through the wafer-sidewall gap122 into the upper process zone 130, a lower zone vacuum pump 216 iscoupled to the lower process zone 132 through a lower zone slit passage218 that is near (but slightly below, by a few mm or less) the waferedge. The upper and lower slit passages 217, 218, are within a few (orseveral) mm of each other along the rotational axis of symmetry of thechamber. In the illustrated reactor of FIG. 2, both slit passages 217,218 are at heights above and below (respectively) the wafer by about 1mm, although this distance may be in a range of about 0.5 to 2 mm, forexample. The slit passage 217, 218 may be axially displaced from oneanother by about 1-2 mm. Generally the distance is less than thewafer-to-ceiling gap (the height of the upper process zone 130). Theupper and lower zone vacuum pumps 210, 216 operate simultaneously toremove through separate slit passages 217, 218 etchant-scavengerreaction by-products (from the upper zone 130 through the slit passage217) and etchant species and etchant-polymer reaction by-products (fromthe lower zone 132 through the slit passage 218).

The slit openings 217, 218 have a narrow (e.g., 0.2-2 mm) axial heightand extend around at least nearly the entire circumference of the sidewall 102. The slit openings 217, 218 are each completely enclosed exceptfor their connections to the respective pumps 210, 216.

In one embodiment, with the upper and lower vacuum pumps 210, 216providing optimum performance, the main vacuum pump 146 is eliminated inthe reactor of FIG. 2. Providing the upper and lower vacuum pumps 210,216, increases the flux of etchant species to the wafer backside.

The upper and lower vacuum pumps 210, 216 and their slit passages 217,218 may also be included in the reactor of FIG. 1A, although they maynot be required in the reactor of FIG. 1A because of the lack of anupper external plasma source 200 in FIG. 1A. Because the upper externalplasma source 200 is not present in the reactor of FIG. 1A, the purgegases may be pumped through the gas distribution plate 104 at very highpressure to protect the wafer front side. Therefore, the local pumps210, 216 and their slit passages 217, 218 are not necessarily requiredin the reactor of FIG. 1A.

Various kinds of plasma sources may be used for the upper and lowerexternal plasma sources 134, 200, such as microwave, conventional ICP ortoroidal. The process chemistries that are to be used in the upper andlower sources 134, 200 limit the choice of materials. Toroidal reactorstypically have metallic chambers or conduits, such as anodized aluminum,which is incompatible with the hydrogen chemistry of the upper plasmasource 200. However, toroidal plasma sources are also available withquartz liners or quartz toroidal shaped (round or square) vacuumvessels. If non-metal, non-coated metal, and non-quartz material isrequired for comparability with the plasma chemistry, then the choice ofexternal plasma source may be more limited to conventional inductivelycoupled plasma sources, such as a quartz, alumina, sapphire or Yittriatube wrapped with an RF-driven coil, for example. Sources may also beelectrostatically shielded to reduce plasma ion bombardment andsubsequent erosion or particle/contamination issues. In one example, thelower external plasma source 134 may be a toroidal plasma source,consisting of a toroidal chamber 220 fed by the process gas supply 136,a coiled RF power applicator 222 coupled to the toroidal chamber 220 anda passage 224 from the toroidal chamber 220 to the port 138. The coil222 may be driven by an RF generator through an impedance match, or maysimply driven by a switched power supply (We need to discuss). Thetoroidal chamber 220 is typically formed of metal with a dielectricexternal film, such as anodized aluminum, which is compatible with theoxygen and nitrogen gases employed in the lower external plasma source134. Because the upper external plasma source 200 is supplied withhydrogen gas, anodized aluminum is not a practical material for theupper source 200, and therefore its chamber 204, in one example, isformed of another material (such as quartz) that is compatible withhydrogen.

In an optional mode, the reactor of FIG. 2 is used to remove photoresistfrom the wafer front side. In this application, oxygen (polymer etchant)flow from the gas supply 136 is halted (or the plasma in the lowersource 134 is extinguished). Preferably, this step is carried out withthe height of the upper process zone 130 being in the narrow regime(0.2-2 mm) discussed above, to enhance the photoresist removal rate.Alternatively, uniformity of the photoresist removal may be enhanced byincreasing the upper process zone height, in which case the waferpedestal 112 is lowered to the dashed-line position of FIG. 2, toenlarge the upper process zone 130 with a wafer-to-ceiling gap of about2.5 to 5 cm. Hydrogen radicals or related plasma by-products from theupper external plasma source 200 fill the upper process zone 130 andremove photoresist from the wafer front side in a reactive etch process.This reaction is promoted by supplementing the hydrogen gas flow to theupper external plasma source 200 with an oxygen-containing gas (H2O orN2O) at a lower flow rate from another gas supply 240. The flow rate ofthe oxygen-containing gas into the upper plasma source 200 may be lessthan 5% of the hydrogen flow rate. This photoresist removal step may beperformed before or after the backside polymer removal step.

In an alternative embodiment of the optional frontside photoresistremoval mode, a capacitively coupled plasma is generated from thehydrogen in the upper process zone 130 by an RF power generator 250coupled through an impedance match 252 across the ceiling gasdistribution plate 104 and the wafer support pedestal 112 (in itslowered dashed-line position of FIG. 2). In this embodiment, hydrogenions are generated in the upper process zone 130 to conduct reactive ionetching of the photoresist on the wafer front side.

The initial placement of the reactor of FIG. 2 in a plasma etch systemmay require replacing one of two single wafer load locks normallypresent in such a system with the reactor of FIG. 2. An etch systemtypically includes four plasma etch reactors, two single wafer loadlocks and a factory interface. For greater versatility, the reactor ofFIG. 2 may be configured to perform the functions of the single waferload lock that it replaces in the plasma etch system. For this purpose,wafer ingress/egress slit valves 270, 272 are provided on opposite sidesof the reactor through the side wall 102 (and liner 120). The pair ofslit valves 270, 272 enables the reactor of FIG. 2 to function as asingle wafer load lock.

Referring to FIG. 3, in one embodiment, a localized stream or jet ofplasma radicals, neutrals, and ions of an etchant species is used toprovide an even higher rate of backside polymer removal. The stream orjet of plasma species is directed onto a small target area or window ofthe wafer backside edge while rotating the wafer. For this purpose, alocal external plasma source 300 may be located near the wafer edge, anda short conduit 302 directs plasma ions from the interior of the localexternal plasma source 300 as a localized stream of plasma ions,radicals and neutrals to the small target region at the wafer backsideedge. In one embodiment, the conduit 302 is sufficiently short and itsoutput end is sufficiently close to the wafer backside to enable toenable ions from the source 300 to reach the wafer backside. Forexample, the transit distance between the output end of the shortconduit and the wafer support plane of the pedestal 112 may be 5% orless of the pedestal or wafer diameter. A gas supply 136 furnishes apolymer etch precursor gas to the local external plasma source 300. Inone embodiment, to expose the entire backside periphery or edge of thewafer to the localized plasma stream, the wafer pedestal 112 is rotatedby a rotation actuator 304 coupled to the support member or leg 114 ofthe pedestal 112. By operating the local external plasma source 300 at alow chamber pressure, it becomes a rich source of plasma ions andelectrons, and the concentrated stream from the conduit 302 consists ofa large proportion of ions in an ion/radical mixture. By placing theexternal plasma source 300 close the wafer 118 and keeping the conduit302 short, ion loss through recombination is minimized, and the particlestream emanating from the conduit 302 remains rich in ions.

In one embodiment, the backside polymer etch rate is increased by theion jet stream in the reactor of FIG. 3; the ion energy at the waferbackside edge surface may be increased by applying RF bias power betweenthe local external plasma source 300 and the wafer support pedestal. Forthis purpose, an RF bias power generator 310 is coupled through animpedance match element 312 across the wafer support pedestal and thelocal external plasma source 300. A gas supply 156 can furnish to thelocal external plasma source 300 a dissociation agent gas (e.g.,nitrogen) that promotes the dissociation of the etchant species (e.g.,oxygen) in the plasma of the local external plasma source 300.

If a radical stream is desired rather than an ion stream, then thechamber pressure in the local external plasma source may be increased.Raising the chamber pressure of the local external plasma source 300reduces the proportion of ions and raises the proportion of radicals inthe stream of particles ejected by the conduit 302. Furthermore, if apurely radical stream is required, then it is not necessary to locatethe external plasma source 300 near the wafer. Instead, it may belocated (for example) near the bottom of the main chamber 100 (asindicated in dashed line in FIG. 3) and the conduit 302 may berelatively long (as shown in dashed line in FIG. 3). The concentratedjet stream from the plasma source 300 can be very hot (e.g., as much as600 degrees C.), and this heat can expedite the reaction between theetch species and the backside polymer. Preferably, the entire wafer isinitially heated to about 300 degrees C. before exposing the backside tothe concentrated radical or ion stream from the local plasma source 300and conduit 302.

Referring to FIG. 4, special precautions may be taken to minimize metalcontamination from the ion stream emanating from the plasma source300/conduit 302. Specifically, metal surfaces can be protected with adielectric (e.g., quartz) liner 320 that covers the bottom surface ofthe ceiling gas distribution plate 104 and a dielectric liner 120 thatcovers the interior surface of the side wall 102. The enlarged view ofFIG. 4 shows how each slit opening 217, 218 may open into a largerpassage, but is completely enclosed within the chamber wall except forthe connection to the respective pump 210, 216.

FIG. 5 depicts an exemplary method that can be carried out in thereactor of FIG. 1A. A first step (block 402) is to support the wafer(using the pedestal 112) so as to expose a peripheral portion of thewafer back side, while beginning to heat the wafer to on the order of300 degrees C. A next step (block 404), which may begin before the finalwafer temperature (e.g., 300 degrees C.) is reached, is to define anupper process zone 130 above the wafer front side and a lower processzone 132 below the wafer back side with minimal migration of gas betweenthe two zones by maintaining a wafer-to-sidewall gap at less than 2 mm.This gap should be sufficiently small to produce a gas flow resistancethat exceeds that of other portions of the chamber by a factor on theorder of 100. A further step (block 406) is to prevent accumulation ofetch species at the wafer front side, or (equivalently) facilitate fastevacuation of the upper zone 130, by maintaining a wafer-to-ceiling gap(the height of the upper process zone 130) at a value at which a highgas flow resistance is established, e.g., less than 2 mm. This gapshould be sufficiently small to confine the upper process zone 130 to across-sectional aspect ratio greater than on the order of 100. Anotherstep (block 408) is to generate a plasma in an external plasma chamber134 with a polymer etchant precursor gas (e.g., oxygen), and introduceby-products (e.g., radicals, free oxygen) from the plasma into the lowerprocess zone 132 so as to etch polymer from the wafer back side. Arelated step (block 410) is to enhance dissociation of the polymeretchant precursor species (oxygen) by introducing a dissociation agent(nitrogen gas) into the external plasma chamber 134. In order to avoidor minimize etching thin films on the wafer front side, a further step(block 412) consists of reducing the amount of polymer etchant precursorspecies (oxygen) in the upper process zone 130 by injecting a purge gas(e.g., N2 or Ar) into the upper process zone 130. A related step (block414) consists of further reducing etchant species in the upper processzone 130 by introducing into the upper process zone a scavenger gas(e.g., H2 or CO) that scavenges the etch species (e.g., oxygen). Thescavenger gas may be used in addition to or instead of the non-reactivepurge gas.

FIG. 6 depicts an exemplary method that can be carried out in thereactor of FIG. 2. A first step (block 416) is to support the wafer onthe pedestal 112 so as to expose a peripheral portion of the wafer backside while heating the wafer to on the order of 300 degrees C. A nextstep (block 418) is to define an upper process zone 130 above the waferfront side and a lower process zone 132 below the wafer back side withminimal migration of gas between the two zones by maintaining awafer-to-sidewall gap at less than 2 mm. A further step (block 420) isto prevent accumulation of etch species or plasma at the wafer frontside by maintaining a wafer-to-ceiling gap (the height of the upperprocess zone) at less than 2 mm. Another (block 422) step is to generatea first plasma in a lower external plasma chamber 134 with a polymeretchant precursor gas (e.g., oxygen), and introduce by-products (e.g.,radicals, free oxygen) from the plasma into the lower process zone 132so as to etch polymer from the wafer back side. A related step (block424) is to enhance dissociation of the polymer etchant precursor species(oxygen) by introducing a dissociation agent (nitrogen gas) into thelower external plasma chamber, at a flow rate of 1-10% of the oxygen gasflow rate. Another step (block 426) is to generate a second plasma in anupper external plasma chamber 200 with a scavenger species (H2 or N2)that scavenges the polymer etch species, and introduce by-products ofthe second plasma (H radicals or N radicals) into the upper process zone130. In order to reduce or eliminate etching of thin films on the waferfront side, a further step (block 428) is to evacuate the upper processzone 130 at a pumping port 217 near the wafer edge to remove polymeretchant species (oxygen) from the upper process zone at a sufficientlyhigh rate to avoid damage of critical (carbon-containing or low-k) filmson the wafer front side. A related step (block 430) is to evacuate thelower process zone 132 at a pumping port 218 near the edge of the waferat a sufficiently high rate to minimize migration of polymer etchantspecies (oxygen) from the lower process zone 132 into the upper processzone 130 and to maximize delivery of polymer etchant species to thebackside edge of the wafer. For maximum delivery of polymer etch speciesto the wafer backside edge, only the front side and backside pumps 210,216, are used, the main pump 146 being eliminated or unused.

FIG. 7 depicts an exemplary method carried out in an optional mode ofthe reactor of FIG. 2, in which the reactor is employed to etchphotoresist from the wafer front side. A first step (block 432) is tostop the flow of polymer etchant species (oxygen) from the lowerexternal plasma source 134 to the lower process zone. Although it ispreferable to continue to restrict the height of the upper process zone130, optionally this height may be increased in preparation for thefrontside photoresist removal step, in which case the next (optional)step (block 434) is to increase the wafer-to-ceiling gap to a distance(e.g., 0.5-5 cm) at which etchant species can accumulate in the upperprocess zone 130. However, it is not necessarily required to increasethe upper process zone height in order to perform photoresist strippingon the wafer front side. The next step (block 436) is to introduce at areduced flow rate an oxygen-containing species (H2O or N2O) along withthe H2 gas into the upper external plasma source 200 (at a flow rate ofless than 1-10% of the hydrogen gas flow rate) so as to enhance the etchrate of photoresist from wafer front side. The process of FIG. 7 may beperformed either before or after the process of FIG. 6. The removal rateof photoresist in this step is enhanced if the height of the upperprocess zone 130 is restricted to the narrow (0.2-2 mm) range. In theother hand, uniformity is enhanced by increasing this height, and thestep of block 434 may only increase the upper process zone height by afractional amount.

FIG. 8 depicts an alternative method for the reactor of FIG. 2 to etchphotoresist from the wafer front side, in which a capacitively coupledplasma is generated in the upper process zone 130. A first step (block438) is to stop the flow of polymer etchant species (oxygen) from thelower external plasma chamber 134 to the lower process zone, and then(block 440) increase the wafer-to-ceiling gap to about 2 to 5 cm. A nextstep (block 442) is to introduce a photoresist removal species gas (H2)into the upper process zone 130. A further step (block 444) is tointroduce at a reduced flow rate an oxygen-containing species (H2O orN2O) into the upper process region. This reduced flow rate may be about1-10% of the hydrogen gas flow rate. A next step (block 446) is to applyRF power into the upper process zone to produce a plasma that removesphotoresist from the wafer front side.

FIG. 9 depicts an exemplary method that can be carried out in thereactor of FIG. 3, in which backside polymer is removed by aconcentrated or localized stream of plasma ions, radicals and neutralsfrom an external plasma source. A first step (block 448) is to supportthe wafer on the pedestal 112 so as to expose a peripheral portion ofthe wafer back side while heating the wafer to on the order of 300degrees C. A next step (block 450) is to define an upper process zone130 above the wafer front side and a lower process zone 132 below thewafer back side with minimal migration of gas between the two zones bymaintaining a wafer-to-sidewall gap at less than 2 mm. A further step(block 452) is to prevent (or minimize) flow or delivery rate of etchspecies or plasma at the wafer front side by maintaining awafer-to-ceiling gap (the height of the upper process zone) at less than2 mm. A further step (block 454) is to generate a first plasma in alocal external plasma chamber 300 with a polymer etchant precursor gas(e.g., oxygen), and direct a narrow stream of by-products from the firstplasma through an injection orifice 302 near the wafer backside directlyat the wafer back side, while rotating the wafer. A related step (block456) is to enhance dissociation of the polymer etchant precursor species(oxygen) by introducing a dissociation agent (nitrogen gas) into thelocal external plasma chamber 300. Another related step (block 457) isto enhance the backside polymer etch rate by applying RF bias poweracross the local external plasma chamber and the wafer. Another step(block 458) is to generate a second plasma in an upper external plasmasource 200 with a scavenger species (H2 or N2) that scavenges thepolymer etch species, and introduce by-products of the second plasma (Hradicals or N radicals) into the upper process zone 130. Another step(block 460) is to evacuate the upper process zone 130 at a pumping port212 near the wafer edge to remove polymer etchant species (oxygen) fromthe upper process zone at a sufficiently high rate to avoid damage ofcritical (carbon-containing or low-k) films on the wafer front side. Arelated step (block 462) is to evacuate the lower process zone 132 at apumping port 218 near the edge of the wafer at a sufficiently high rateto minimize migration of polymer etchant species (oxygen) from the lowerprocess zone 132 the into the upper process zone 130.

FIG. 10 depicts an exemplary process carried out in the reactor of FIG.2 for simultaneously removing polymer from the wafer backside whileremoving photoresist from the wafer front side. The process of FIG. 10may be carried out, for example, in cases in which there is no film onthe wafer front side (such as a ULK film) that is particularlysusceptible to damage from polymer etchant species, or in cases in whichany critical or ULK films that are present can tolerate the limited flowof polymer etch species escaping from the lower process zone 132 to theupper process zone 130. A first step (block 464) is to support the waferso as to expose a peripheral portion of the wafer back side whileheating the wafer to on the order of 300 degrees C. A next step (block466) is to define an upper process zone 130 above the wafer front sideand a lower process zone 132 below the wafer back side with minimalmigration of gas between the two zones by maintaining awafer-to-sidewall gap at less than 2 mm. A further step (block 468) isto generate a first plasma in a lower external plasma chamber 134 with apolymer etchant precursor gas (e.g., oxygen), and introduce by-products(e.g., radicals, free oxygen) from the plasma into the lower processzone 132 so as to etch polymer from the wafer back side. A related step(block 470) is to enhance dissociation of the polymer etchant precursorspecies (oxygen) by introducing a dissociation agent (nitrogen gas) intothe lower external plasma chamber 134. A step (block 472) carried outsimultaneously with the step of block 468 is to generate a second plasmain an upper external plasma chamber 200 with a scavenger precursor gas(H2) and a small proportion of an oxygen-containing species (H2O orN2O). By-products of the second plasma, e.g., scavenger species (Hradicals) and oxygen-containing radicals, are introduced into the upperprocess zone 130. The scavenger species removes etch species (e.g.,oxygen) from the upper process zone 130, and—with the help of theoxygen-containing species—removes photoresist from the wafer front side.During this process, it is preferable to maintain the height of theupper process zone 130 in the narrow range (0.2-2 mm) to enhance therate of photoresist removal. The result of this step is tosimultaneously remove backside polymer using the bottom external plasmasource 134 and remove front side photoresist with the upper externalplasma source 200. The flow rate of the oxygen-containing (H2O or N2O)into the upper external plasma source 200 species may be 1-10% of thehydrogen flow rate into the upper external plasma source 200, forexample.

FIG. 11 depicts a variation of the reactor of FIG. 3, in which theplasma by-product stream or jet directed at the wafer backside edge isproduced by components within the reactor itself rather than an externalplasma source. For this purpose, an external plasma source such as theexternal plasma source 300 shown in FIG. 3 is replaced in the embodimentof FIG. 11 by an internal plasma source 500 consisting of a cylindricalsealed enclosure 502, which may be formed of an insulating material suchas quartz, and a coil 504 wrapped around a portion of the cylindricalenclosure 502. In the illustrated reactor, the coil 504 is outside ofthe chamber. The enclosure 502 is closed at its bottom end 502 a andforms a nozzle or conical shaped outlet 502 b at its top end facing andclose to the backside edge of the wafer 118. A gas supply 506 storing apolymer etch gas species is coupled to the enclosure 502 through thebottom end 502 a. An RF generator 508 is coupled to the coil 504(through an optional impedance match, not shown) and furnishessufficient power to produce an inductively coupled plasma inside theenclosure 502. The pressure inside the enclosure 502 is greater thanthat of the lower process zone 132. This pressure difference may becontrolled by the main vacuum pump 146 and an optional vacuum pump 509coupled to the enclosure 502. Plasma by-products, for example radicals,neutrals and/or ions, escape through the nozzle outlet 502 b and form aconcentrated or localized stream 510 that impinges upon a target area ofthe wafer backside edge. In order to control the ion energy of thestream 510, an optional RF bias power generator 512 may be connectedbetween an interior electrode 514 inside the enclosure 502 and the waferpedestal 112. While the upper external plasma source 200 of FIG. 3 maybe employed in the reactor of FIG. 11, this option is depicted in FIG.11 only in dashed line. Instead, the solid line image of FIG. 11 showsthat (optionally) the purge gas supply 140 may furnish purge gasesthrough the ceiling gas distribution plate 104 with no external plasmasource. The purge gas may be either non-reactive or may be a reactivescavenger species, as discussed with reference to FIG. 1.

In one embodiment, to provide a desired (e.g., 300 degree C.) wafertemperature, the pedestal 112 may be heated as in FIG. 3, or radiantlamps (not shown) above the ceiling may be employed. In one embodiment,a liner 520 of a process-compatible material may cover the side andbottom edge surfaces of the pedestal 112, and a liner 522 of a processcompatible material may cover the side wall 102. The liners 520, 522 maybe useful in minimizing metal contamination due to etching of chambersurfaces by the plasma stream 510. The process-compatible material maybe, for example, quartz. In one implementation, the ceiling 104 may beformed of a process-compatible material such as quartz. In this case,the ceiling may be a smooth simple structure without the gasdistribution plate features depicted in FIG. 11.

In an alternative embodiment, the plasma source enclosure 502 of FIG. 11may be in the shape of a torus, to form a toroidal plasma source.

In another alternative embodiment, the inductive plasma source 500 (thetube enclosure 502) is replaced by a capacitively coupled source 530 asillustrated in FIG. 12. The capacitively coupled source 530 includes aconductive electrode 532 having a small discharge portion or area 532-1close to and facing the backside edge of the wafer 118 and at least oneaxially extending leg 532-2. A polymer etch gas species is introducedinto the lower process zone 132 through an opening 533 in the chamberfloor from a gas supply 506. An RF generator 534 is coupled between thebottom end of the axially extending leg 532-2 and the wafer pedestal112. A second axially extending leg 532-3 parallel to the first leg532-2 may be provided. RF power from the generator 534 produces a plasmadischarge in the small gap between the electrode discharge portion 532-1and a corresponding area on the wafer backside edge. In one embodiment,the side of the electrode 532 facing the wafer 118 (or the entireelectrode 532) may be covered by a liner 535, which may be formed of aprocess-compatible material such as quartz, which is useful inminimizing or preventing metal contamination. As in the embodiment ofFIG. 11, the wafer is rotated so that the entire backside edgecircumference is exposed to the localized plasma.

If radiant heating is employed to heat the wafer 118, then the pedestal112 may not be necessary, as will be discussed below.

In the reactor of FIG. 13, the need to rotate the wafer to expose itsbottom circumference to a plasma stream is eliminated by insteadgenerating a ring of plasma 538 beneath the backside edge extendingaround the entire circumference. This is accomplished by placing thewafer 118 very close the ceiling 104, as in the foregoing embodiments,and then applying RF power to a coil antenna 540 overlying the edge ofthe wafer 118. The coil antenna 540 may consist of solenoidal conductivewindings, for example. While the wafer may be held in the elevatedposition depicted in FIG. 13 with the heated pedestal 112 of FIG. 11,FIG. 13 illustrates how the wafer may be elevated by lift pins 542suspended on the lift spider 544 controlled by the lift actuator 116. Inthis case, the wafer is heated by radiant lamps 548 through the ceiling104. Alternatively, the above-ceiling coil 540 may be replaced by a coil540′ around the sidewall 102.

In certain embodiments, photoresist is removed from the wafer front sidein a separate process. In such embodiments, the wafer is lowered to thedashed line position of FIG. 13, and gases are introduced through theceiling gas distribution plate 104 capable of removing photoresist, asdiscussed previously in this specification. In other embodiments, asecond inner coil antenna 550 is provided controlling plasma ion densitynear the center of the wafer. The presence of the second inner coilantenna 550 also improves uniformity of the photoresist removal. The twocoil antennas 540, 550 may be driven at independently adjusted RF powerlevels, to permit tuning of the plasma ion density radial distribution.This may be accomplished by providing separate RF generators 552, 554coupled to the separate coil antennas 540, 550, or by providing a singleRF generator 556 whose power is controllably apportioned between the twoantennas 540, 550 by a power splitter 558. The lamp heaters 548 areplaced in open spaces over the ceiling between the inner and outer coilantennas 540, 550.

The heated pedestal 112 of FIG. 11 can be used to hold the wafer 118 inthe elevated position of FIG. 13. In this case, for a low chamberpressure and in the absence of radiant lamp heaters, efficient heattransfer requires the use of an electrostatic chuck on the pedestal 112.An advantage of using the wafer support pedestal is that it enables biaspower to be applied to the wafer in a highly uniform manner while at thesame time effecting uniform heating or temperature control of the wafer.A liner 520 can also be provided having process-compatible materials toavoid metal contamination and excessive consumption of pedestalmaterials during plasma processing.

An advantage of using the radiant lamps 548 to heat the wafer is fasterheat transfer (compared to a heated pedestal) in the case of a lowchamber pressure where heat conduction or convection is poor.

FIG. 14 depicts another embodiment of the reactor of FIG. 13 in whichthe coil antenna 540 for generating a ring plasma is replaced by a ringelectrode 560 surrounding the wafer edge, and an RF generator 562coupled to the ring electrode 560. RF discharge from the ring electrode560 produces the ring plasma 538 by capacitive coupling. FIG. 15 depictsa modification of the reactor of FIG. 14 in which a ring electrode 560′is outside of the chamber 100.

FIG. 16 depicts a modification that can be implemented in any of thereactors described herein, in which the ceiling 104 has a shallowcylindrical hollow 570 corresponding to the volume of the wafer 118. Incertain embodiments, the wafer can be lifted into the hollow 570 toeffectively shield the wafer front side from polymer etch gases duringthe backside polymer removal process,

FIG. 17 illustrates a process in which the reactor chamber is used toperform reactive ion etch of polymer from the wafer backside in a raisedwafer position and then photoresist stripping on the wafer front side ina lowered wafer position. Temperature switching is employed to enhancewafer front side protection. Specifically, backside polymer removal isperformed at a low wafer temperature and then front side photoresiststrip is performed at a high wafer temperature (the order may bereversed). The backside polymer reactive ion etch step may be performedat a sufficiently low wafer temperature to retard the reaction of itsby-products (radicals) with thin films (e.g., photoresist) on the waferfront side. If the front side photoresist strip process uses radicalsfrom a remote source, it is facilitated by raising the wafer temperatureto a threshold at which the reaction rate of the radicals withphotoresist is significantly increased. If the front side photoresiststrip process is a reactive ion etch process, then the wafer temperaturedoes not necessarily have to be increased for this step.

Referring now to an exemplary process shown in FIG. 17, a first step(block 600) is to support the wafer on the pedestal so as to expose aperipheral portion of the wafer back side. The wafer temperature is setunder a threshold temperature (e.g., under 200 degrees C.) below whichthe reaction rate of polymer etch species radicals with wafer thin filmmaterials is significantly retarded (block 602). A next step (block 604)is to define an upper process zone above the wafer front side and alower process zone below the wafer back side with minimal migration ofgas between the two zones by maintaining a wafer-to-sidewall gap at lessthan 2 mm. A further step (block 606) is to prevent accumulation of etchspecies or plasma at the wafer front side by maintaining awafer-to-ceiling gap (the height of the upper process zone) at less than2 mm. Another step (block 608) is to purge the upper process zone toremove any etch species radicals that may leak through thewafer-sidewall gap. In order to reduce or avoid etching of thin films onthe wafer front side, a further step (block 610) is to evacuate theupper process zone at a pumping port near the wafer edge to removepolymer etchant species (oxygen) from the upper process zone at asufficiently high rate to avoid damage of critical (carbon-containing orlow-k) films on the wafer front side. A related step (block 612) is toevacuate the lower process zone at a pumping port near the edge of thewafer. Reactive ion etching of the polymer on the wafer backside isperformed with a plasma close to the wafer backside edge (block 614),until the backside polymer is completely removed. Application of plasmato the wafer backside is then stopped, and the wafer is lowered toincrease the height of the upper process zone, in order to permitaccumulation of plasma or radicals in the upper process zone (block616). Then, the wafer temperature is increased above a higher thresholdtemperature (e.g., above 300 degrees C.) in order to significantlyincrease the reaction rate of the radicals with photoresist on the waferfront side (block 618). Typically, the reaction rate increasecorresponding to the temperature increase from the lower thresholdtemperature to the higher threshold temperature is about a factor of 5.Radicals from a remote plasma source are employed to strip the frontside photoresist (block 620). Alternatively, the step (block 618) ofraising the wafer temperature may be omitted, and a reactive ion etchprocess is used to strip the photoresist (block 622).

FIG. 18 depicts a modification of the plasma reactor of FIG. 13 that iscapable of performing the process of FIG. 17. In this modification, afront side (e.g., photoresist strip) process gas supply 160 is providedin addition to the purge gas supply 140 through the ceiling 104. Theouter coil 540′ is moved to an axial location below the wafer plane.Optionally, the heater lamps 548 are moved from the ceiling to the floorat the bottom of the chamber. The heater lamps 548 of FIG. 18 may beemployed in carrying out the wafer temperature control step of block 602of FIG. 17. A quartz window 549 is provided in the floor for the heatlamps 548. The small wafer-ceiling gap of block 606 of FIG. 17 isrealized at the raised (solid line) position of the wafer in the reactorof FIG. 18. The purge gas supply 140 of FIG. 18 provides the gas for thepurge step of block 608 of FIG. 17. The pumping ports 217 and 218 ofFIG. 18 are used in the steps of blocks 610 and 612 of FIG. 17. Thereactive ion etch step of block 614 of FIG. 17 is carried out by thesplitter 558 of FIG. 18 applying RF power only to the outer coil 540′.This creates a ring of plasma beneath the backside wafer edge, asdescribed previously with reference to FIG. 13, to remove the backsidepolymer. Upon completion of this step, the wafer 118 is lowered to thedashed line position of FIG. 18, and process gas (e.g., a photoresiststrip process gas) is introduced through the ceiling 104 from the gassupply 160 and into now enlarged upper process zone. The splitter 558applies RF power to both the inner and outer coils 540, 550, with powerbeing apportioned between the two coils to optimize plasma ionuniformity over the front side of the wafer. The RF power and gas flowis maintained until the completion of the reactive ion photoresist etchstep of block 622 of FIG. 17.

FIG. 19 depicts a modification of the embodiment of FIG. 18, in whichthe ceiling 104 is modified to form an upward extending neck 650 havinga cylindrical side wall 652 and a neck cap 654, forming a neck volume656. The outputs from the purge gas supply 140 and the process gassupply 160 are received in the neck volume. The inner coil 550 is woundaround the neck sidewall 652, so that the neck volume 656 functions asthe chamber of a remote plasma source. The splitter 558 of FIG. 19applies power only to the outer coil 540 during the backside etchprocess to form a ring of plasma beneath the wafer backside edge. Thesplitter 558 applies power only to the inner coil 550 during the frontside etch process to provide a remote plasma source. Because of thedistance of the remote source chamber 656 from the wafer 118 in itslowered (dashed line) position of FIG. 19, ions from the plasma in theneck recombine before reaching the wafer 118, and therefore the frontside etch is a radical-based process, in accordance with the step ofblock 620 of FIG. 17. This makes it advantageous to use the heater lamps548 of FIG. 19 prior to this step to increase the wafer temperatureabove the radical reaction threshold temperature, in accordance withblock 618 of FIG. 17.

The reactor of FIG. 11 may be adapted to perform the process of FIG. 17by adding a remote plasma source 200 and its process gas supply. Theremote plasma source 200 may be implemented with a coil wrapped around aneck or tube (as shown in FIG. 19) or it may be any other type of plasmasource, such as a microwave plasma source, for example. The waferpedestal 112 of FIG. 11 may control the wafer temperature in accordancewith the process of FIG. 17, and the pedestal 112 may move between theraised and lowered (dashed-line) positions of FIG. 11, as an optionalfeature of the process of FIG. 17. The reactor of FIG. 12 is modified inthe same manner to adapt it to perform the process of FIG. 17, by addingthe remote plasma source 200 at the ceiling of the reactor of FIG. 12,and moving the pedestal 112 between the solid and dashed-line positionsof FIG. 12. The same modifications may carried out in each one of thereactors of FIGS. 13, 14 and 15. Each one of FIGS. 13, 14 and 15 showsthat, while lift pins may be employed to control the wafer position, amovable heated pedestal 112 (partially shown in dashed line in both itselevated and retracted positions) may be employed instead, in which casethe heater lamps 548 would not be required. In each of the reactors ofFIGS. 11-15, the optional use of a wafer pedestal 112 enables bias powerto be applied to the wafer. Advantages of this feature include theenhancement of the backside polymer etch process, and the suppression ofions in the upper process zone 130 if the wafer-ceiling gap is less thanthe plasma sheath thickness.

Reactive ion etching for removal of backside polymer from the wafer canbe accomplished in the process of FIG. 9 using the reactor of FIG. 3. Inthis aspect, the backside plasma source 300 of FIG. 3 produces asufficient flow of ions that reach the wafer backside. This condition isrealized by holding the chamber pressure of the external plasma source300 to a low pressure. In this step, the wafer temperature is held undera low threshold temperature (e.g. below about 200 degrees C. below whichthe reaction rate of polymer etch radicals with wafer front side thinfilm materials is very slow (e.g., about 5 times slower than at about300 degrees C.). The reactive ion etch process used to remove thebackside polymer is not hampered at low wafer temperatures. Therefore,this low wafer temperature does not prevent the polymer etch ions fromreacting with the backside polymer, but it does effectively slow downthe reaction rate of polymer etch radicals that may escape from thelower process zone 132 into the upper process zone 130 with the waferfront side materials. The small wafer-to-ceiling gap employed during thebackside polymer removal step essentially guarantees that most (or all)of the polymer etch species that can survive in the upper process zone130 are radicals or neutrals, not ions.

During the front side photoresist strip step of FIG. 7, in which thereare no polymer etch species threatening the wafer front side, the wafertemperature is raised above a high threshold temperature (e.g., above300 degrees C.) at which radicals can react at a faster rate with frontside thin film materials, such as photoresist (e.g., at a rate about 5times faster than at the lower threshold temperature of 200 degrees C.).Typically, the wafer temperature is constantly ramping. If the wafer isbeing heated to ramp its temperature upwardly, then the backside polymerremoval step is performed prior to the wafer temperature exceeding thelower threshold temperature, and the front side photoresist strip stepis not performed until after the wafer temperature has reached orexceeded the higher threshold temperature. For example, the wafertemperature may begin slightly above room temperature, ramps upwardlyduring the entire process. In the period before the temperature exceedsabout 150 degrees C. or 200 degrees C., the backside polymer removalstep is performed and halted upon completion. Then, after a pause toallow the wafer temperature to reach or at least get near 300 degreesC., the front side photoresist step is performed. The processes of FIGS.7 and 9 may be performed in any order, an advantage being that they areperformed in the same reactor without having to remove the wafer. Whenlowering the wafer as required in the process of FIG. 7, the backsideplasma source (300 of FIG. 3 for example) is moved so as not to obstructwafer movement.

The reactor of FIG. 3 may be modified by replacing the external plasmasource 300 with an internal plasma source, such as the internalinductive source 500, 502, 508 of FIG. 11 or the internal capacitivesource 530, 532, 534 of FIG. 12. In this case,

As previously described in detail in this specification, wafer frontside protection is provided by establishing the narrow wafer-ceiling gapand pumping a purge or scavenger gas through that narrow gap. Suchprotection is improved upon by the feature of holding the wafertemperature below the threshold temperature during backside polymerremoval.

Typically, polymer etch radicals (in the absence of ions) react veryslowly with photoresist below the low threshold temperature of about 200degrees C., while a higher reaction rate is obtained above a highthreshold temperature of about 300 degrees C. The polymer etch rateincreases by about a factor of 5 as the wafer temperature increases fromthe low threshold temperature (e.g., 300 degrees C.) and the highthreshold temperature (e.g., 200 degrees C.). Therefore, wafer frontside protection is enhanced by holding the wafer temperature below thelow threshold temperature of 200 degrees C. (e.g., at 150 degrees C.)during the backside polymer removal process. During the subsequent frontside photoresist stripping, the wafer temperature is raised to about 300degrees C.

At a high chamber pressure on the order of a Torr, the heatedelectrostatic chuck or pedestal 112 of FIG. 3 may be employed to controlthe wafer temperature. One advantage of this is that RF plasma biaspower may be applied to the wafer to enhance processing. A possibledisadvantage of using a heated pedestal 112 is that the pedestal 112 mayrequire a protective liner of a compatible material, such as quartz,alumina or yittria. At a lower chamber pressure, it may be necessary touse radiant lamps in order to attain a requisite heat transfer rate. Oneadvantage of radiant lamps is that the wafer temperature may be switchedbetween the two temperatures more quickly, particularly if the wafer islifted above contact with the pedestal 112 during processing.

FIG. 20 illustrates a modification of the reactor of FIG. 2 in which theplasma by-products (radicals) from the external plasma source 134 arefed into an annular plenum 630 inside the bottom of the pedestal 112 andupwardly through axial cylindrical bores 632 within the pedestal 112 andis shot out through ejection orifices 634 aimed generally at theperipheral edge of the wafer 118. The annular plenum 630 supplies gas orplasma byproducts to the bottom of each bore 632 and the ejectionorifices or nozzles 634 receive the plasma by-products from the tops ofthe bores 632. As shown in FIG. 21, the bores 632 and nozzles 634 arearrayed concentrically. In this way, the flow of plasma by-products fromthe external plasma source 134 is collimated within internal bores 632of the pedestal 112 and then aimed as a circular array of directed spraypatterns from the ejection orifices 634. The pedestal 112 has awafer-support surface 636 having a diameter less than that of the wafer118 so as to leave a peripheral annular region of the wafer backsideexposed. The pedestal 112 further has a peripheral annular surface 638that is parallel to but below the wafer support surface, the heightdifference between the two surfaces 636, 638 providing a gas flow space640 between the peripheral annular surface 638 and the wafer backside.

As in the embodiments of FIGS. 1-3, a boundary between the upper andlower process zones 130, 132 is established by constricting gas flow atthe peripheral edge of the wafer 118. In the reactor of FIG. 20, this isaccomplished by providing a confinement ring 642 surrounding thepedestal 112 and having an inner edge 642 a adjacent the edge 118 a ofthe wafer, the two edges 118 a, 642 a being separated by a small gap ofabout 0.5-5 mm. This gap is sufficiently small to limit gas flow betweenthe upper and lower process zones 130, 132 and thereby protect the waferfront side during etching of polymer from the wafer backside edge.Evacuation of polymer etch species is performed by the main vacuum pump146 through a horizontal radial space 640 between the annular peripheralsurface 638 of the pedestal and a bottom surface 644 of the ring 642.This evacuation extends through a vertical axial space 646 between theside wall 648 of the pedestal 112 and a vertical surface 649 of the ring642.

In the implementation depicted in the drawing of FIG. 20, the chamber220 of the external plasma source 134 is a toroid and the RF sourcepower applicator 222 consists of a magnetically permeable ring 222 awrapped around a section the chamber 220 and a coil 222 b wrapped aroundthe ring 222 a and driven through an impedance match 135 by an RF sourcepower generator 137. The chamber 220 is coupled to the plenum 630 via aconduit 224.

While the foregoing is directed to embodiments of the invention, otherand further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A process for removing polymer from a backside of a workpiece,comprising: supporting said workpiece on the backside in a vacuumchamber while leaving at least a peripheral annular portion of thebackside exposed; confining gas flow at the edge of said workpiecewithin a gap at the edge of said workpiece on the order of about 1% ofthe diameter of the chamber, said gap defining a boundary between anupper process zone containing said wafer front side and a lower processzone containing said wafer backside; providing a polymer etch precursorgas underneath the backside edge of the workpiece and applying RF powerto a region underlying the backside edge of the workpiece to generate afirst plasma of polymer etch species concentrated in an annular ringconcentric with and underneath the backside edge of the workpiece;removing polymer etch species from said upper process zone, whereinremoving polymer etch species from said upper process zone comprisesgenerating a second plasma from a precursor gas of a scavenger of thepolymer etch species, and introducing scavenger by-products from saidsecond plasma into said upper process zone.
 2. The process of claim 1further comprising: evacuating said upper process zone through a slitopening surrounding said upper process zone.
 3. The process of claim 2further comprising: evacuating said lower process zone through a slitopening surrounding said lower process zone near the edge of saidworkpiece.
 4. The process of claim 1 further comprising: confining saidupper process zone between the wafer front side and a ceiling of thereactor to an upper process zone height on the order of about 1% of thediameter of the chamber.
 5. The process of claim 4 wherein said gap andsaid upper process zone height are between about 0.5 and 5 mm.
 6. Theprocess of claim 1 wherein said polymer etch precursor gas comprisesoxygen gas and said scavenger precursor gas comprises hydrogen ornitrogen gas.
 7. The process of claim 6 further comprising promotingdissociation of oxygen in said first plasma by adding nitrogen gas topolymer etch precursor gas.
 8. The process of claim 1 further comprisingapplying and RF bias between said first plasma and said workpiece.
 9. Aprocess for removing polymer from a backside of a workpiece, comprising:supporting said workpiece on the backside in a vacuum chamber whileleaving a peripheral annular portion of the backside exposed, saidworkpiece corresponding to a boundary between an upper process zone anda lower process zone of said chamber; confining the front side of saidworkpiece by a ceiling of said chamber to as to establish a height ofthe upper process zone that is less than about 1% of the diameter ofsaid workpiece; providing a polymer etch precursor gas underneath thebackside edge of the wafer and applying RF power to a region underlyingthe backside edge of the wafer to generate a first plasma of polymeretch species concentrated in an annular ring concentric with andunderneath the backside edge of the wafer; removing polymer etch speciesfrom said upper process zone, wherein removing polymer etch species fromsaid upper process zone comprises generating an external plasma from aprecursor gas of a scavenger of the polymer etch species, andintroducing scavenger by-products from said second plasma into saidupper process zone.
 10. The process of claim 9 wherein applying RFsource power comprises applying RF power to a ring-shaped RF powerapplicator that is concentric with said workpiece.